J. Feuchtwanger

Magnetic moment and chemical order in off-stoichiometric Ni–Mn–Ga ferromagnetic shape memory alloys

Recent studies have shown that the total magnetic moment in off-stoichiometric Ni–Mn–Ga alloys depends not only on electronic concentration but also on the degree of chemical order in the alloy. We have performed neutron diffraction experiments and magnetization measurements for determining the preferential atomic order and saturation moment in off-stoichiometric compounds (44–52 at.% Ni), having excess Mn and deficient in Ga. These alloys include isoelectronic alloys with different magnetic moments and were chosen in an effort to study the impact of chemical order on the magnetic moment distribution. In this work, we present an improved model of magnetic interaction between Mn atoms, which carry most of the localized magnetic moment of the alloys. The Mn atoms at Ga sites, which are nearest neighbors to properly sited Mn, couple antiferromagnetically to the dominant moment. In contrast, Mn atoms at Ga sites, which are nearest neighbors to Mn at Ni sites, couple ferromagnetically. Mn at Ni sites is always antiferromagnetic (AF). The new model is supported by the exchange variation with the Mn–Mn distance and demonstrates excellent agreement between experimental and calculated magnetic moments. The proposed model is shown to better explain the observed experimental results as compared to the rigid band model and previous localized moment models that assumed AF coupling for all off-site Mn atoms.

Ferromagnetic shape memory alloys for positioning with nanometric resolution

Ferromagnetic shape memory alloys are promising active elements for actuators. Our work centered on achieving and maintaining an intermediate fixed deformation so that they can be used as precision positioning actuators. For this purpose, a custom actuator was built using a single crystal of NiMnGa. The results show that these alloys can be controlled within less than 5 nm for both precision and accuracy, a result comparable to piezoelectric ceramics. Interestingly, the defect structure plays a fundamental role in achieving such performance. The stochastically distributed defects determine a progressive diminution of the magnetic field strength required to achieve the control.

ESS-Bilbao light-ion linear accelerator and neutron source: design and applications

The baseline design for the ESS-Bilbao light-ion linear accelerator and neutron source has been completed and the normal conducting section of the linac is at present under construction. The machine has been designed to be compliant with ESS specifications following the international guidelines of such project as described in Ref. [1]. The new accelerator facility in Bilbao will serve as a base for support of activities on accelerator physics carried out in Spain and southern Europe in the frame of different ongoing international collaborations. Also, a number of applications have been envisaged in the new Bilbao facility for the outgoing light ion beams as well as from fast neutrons produced by low-energy neutron-capture targets, which are briefly described.

Influence of Magnetic Field on Magnetostructural Transition in Ni46.4Mn32.8Sn20.8 Heusler Alloy

A Mn-enriched Ni46.4Mn32.8Sn20.8 Heusler alloy has been prepared by the induction melting and casting method. The samples used were annealed at 900°C during 72 h with a subsequent water quench. The magnetostructural transformation (MST) in the vicinity of Tm=230K and Ta=250K has been found, and characterized by DSC, magnetization and resistivity measurements. Changes in the crystal structure due to the MST were observed using neutron diffraction at different temperatures. The crystal structure of martensitic phase shows a good fits to the 4-layered orthorhombic martensite with a Pmma space group. The linear shift of transformation temperatures was found in the entire high magnetic field range up 14T to be dTm/dB0 = -1.50 K/T and dTa/dB0 = -1.26 K/T, whereas in the low field range we found an increase of both characteristic temperatures. The entropy changes at the MST were evaluated using the Clausius-Clapeyron relationship. The transformation-induced magnetoresistance has been measured.

Magnetic field effect on premartensitic transition in Ni-Mn-Ga alloys

Extensive measurements of the temperature dependencies of resistivity under constant magnetic fields made it possible to clarify the premartensitic transition sensitivity to applied magnetic field up to 14 T in polycrystalline and monocrystalline Ni–Mn–Ga alloys with compositions close to the stoichiometric one. A low-field minimum on the transition temperature versus magnetic field curves is found in both alloys. This minimum is attributed to the magnetic anisotropy contribution to the Clausius–Clapeyron relationship, which describes the premartensitic transition. This contribution is confirmed by magnetization loops measurements.

Very High Sensitivity Displacement Sensor Based on Resonant Cavities

A new way of sensing displacement for subnanometric applications using RF resonant cavities is presented and experimentally demonstrated on a laboratory prototype. The new sensor physical principle of operation is summarized, and a first working prototype is described and tested. It is shown by experiment that very high and linear sensitivity characteristic curves, in the range of several kilohertz per nanometer, are easily obtainable using this kind of devices, making them serious candidates as alternative and reasonably priced sensors for subnanometric displacement measurement.

ISHN Ion Source Control System. First Steps Toward an EPICS Based ESS-Bilbao Accelerator Control System

ISHN (Ion Source Hydrogen Negative) consists of a Penning type ion source in operation at ESS-Bilbao facilities. From the control point of view, this source is representative of the first steps and decisions taken towards the general control architecture of the whole accelerator to be built. The ISHN main control system is based on a PXI architecture, under a real-time controller which is programmed using LabVIEW. This system, with additional elements, is connected to the general control system. The whole system is based on EPICS for the control network, and the modularization of the communication layers of the accelerator plays an important role in the proposed control architecture.

Magnetic moment distribution in non-stoichiometric Ni-Mn-Ga ferromagnetic shape memory alloys

In this work we present a model for the net magnetic moment per formula unit measured in Ni-Mn-Ga Ferromagnetic Shape Memory Alloys, analyzing the different types of magnetic interactions appearing in such compounds. We have studied four compositions ranging from a 44% to a 52% Ni content. We found that excess Mn atoms occupy empty positions at Ni and Ga sites. We consider that the Mn atoms at Ni sites (Mn/Ni) can determine that Mn/Ga atoms couple either ferromagnetic (FM) or antiferromagnetically (AF) to Mn/Mn atoms. If Mn/Ga are nearest neighbors to properly-sited Mn/Mn atoms they present AF coupling. However, when Mn/Ni atoms are at closer distances than Mn/Ga ones, so that they become nearest neighbors to Mn/Mn, the AF coupling between Mn/Ni and Mn/Mn atoms is stronger and overcomes that between Mn/Ga and Mn/Mn ones. Therefore Mn/Ga and Mn/Mn will couple ferromagnetically. Polarized neutron diffraction experiments will help to elucidate the validity of our model.

Stress-Induced Twin Boundary Motion in Particulate Ni-Mn-Ga /Polymer Composites

Composites of Ni–Mn–Ga particles in a polyurethane matrix can be made by mixing the particles with the polymer, and allowing them to cure under a magnetic field to texture the composites. These composites show large hysteresis and mechanical losses, when subjected to a cyclic stress, that were far larger than the matrix polymer ones. The additional losses are attributed to the motion of twin boundaries in the filler particles and provide a way for obtaining mechanical energy absorption in a wide frequency range. By means of X-ray and neutron diffraction we present evidence that confirms that twins are present in the particles and that they do move when mechanically loading the composite

Thin-Film Magnetoimpedance Structures Onto Flexible Substrates as Deformation Sensors

The magnetic and magnetoimpedance (MI) properties of a Permalloy-based thin-film structure, deposited onto a polymeric flexible substrate, are investigated as a function of the applied stress. The selected structure, a sandwich of NiFe/Ti multilayers with a central Cu layer, has been previously shown to produce extraordinary MI performance when deposited onto rigid substrates. Here, grown onto a cyclo olefin polymer substrate, it is evaluated as a deformation sensor and its properties analyzed in terms of the preparation conditions. A specially designed sample holder allows measuring simultaneously the hysteresis loop of the sample using magneto-optical Kerr effect and its MI using a network analyzer, while applying a controlled stress to the sample. The magnetoelastic anisotropy developed in the sample is longitudinal, revealing that the magnetostriction coefficient is positive, and competes with the transverse magnetic anisotropy induced during the sputtering deposition of the multilayer structure. The combined, effective anisotropy determines the shape of the hysteresis loop and the MI curves. We show how the evolution of the maxima of the MI curves with the applied stress allows a precise determination of the magnetostriction coefficient of the sample. The stress impedance is determined as the impedance change, measured at